Environmental concerns have led to continued efforts to reduce the CO, hydrocarbon and nitrogen oxide (NOx) emissions of compression ignited (diesel) internal combustion engines. To meet the latest and future standards heavy duty diesel (HDD) original equipment manufacturers (OEMs) rely on one of two technologies, or a combination thereof. One method used to reduce the emissions of diesel engines is known as exhaust gas recirculation or EGR. EGR reduces NOx emissions by introducing non-combustible components (exhaust gas) into the incoming air-fuel charge introduced into the engine combustion chamber. This reduces peak flame temperature and NOx generation. In addition to the simple dilution effect of the EGR, an even greater reduction in NOx emission is achieved by cooling the exhaust gas before it is returned to the engine. The cooler intake charge allows better filling of the cylinder, and thus, improved power generation. In addition, because the EGR components have higher specific heat values than the incoming air and fuel mixture, the EGR gas further cools the combustion mixture leading to greater power generation and better fuel economy at a fixed NOx generation level.
EGR equipped engines, particularly cooled EGR equipped engines create a harsh environment for lubricating oil compositions due to greater levels of NOx and sulfur oxide (SOx)-based acids (the latter formed from sulfur introduced primarily by combustion of diesel fuel) and particulate matter that circulates through such engines. The API-CI-4 oil specification was established specifically for lubricating oil compositions for use in cooled EGR equipped HDD engines.
The other major technology being relied on to reduce HDD engine emissions, as used specifically in “ACERT”-type engines manufactured by Caterpillar Inc. (USA), involves adjusting engine timing to provide an early close of the engine exhaust valve; use of a pilot fuel injector(s) upstream of the main fuel injectors to reduce NOx generation; rate shaping of combustion to reduce the peak combustion temperature and reduce NOx generation; forcing an excess of air into the combustion chamber (by use, for example, of one or more turbochargers) to provide the required power output, and the catalytic after-treatment devices, such as devices containing oxidation catalysts to reduce levels of unburned hydrocarbons, carbon monoxide, nitrogen oxide and the soluble organic fraction of particulate matter in the engine exhaust gas. Such oxidation catalysts can become poisoned, and rendered less effective, by exposure to certain elements/compounds present in engine exhaust gasses, particularly by exposure to phosphorus and phosphorus compounds introduced into the exhaust gas by the degradation of phosphorus-containing lubricating oil additives. Further, engines may be provided with aftertreatment devices containing reduction catalysts, which catalysts are sensitive to sulfur and sulfur compounds.
One of the most effective antioxidant and antiwear agents (from both a performance and cost-effectiveness standpoint) used in lubricating oil compositions for internal combustion engines comprises dihydrocarbyl dithiophosphate metal salts. The metal may be an alkali or alkaline earth metal, or aluminum, lead, tin, molybdenum, manganese, nickel or copper. Of these, zinc salts of dihydrocarbyl dithiophosphate (ZDDP) are most commonly used. While such compounds are particularly effective antioxidants and antiwear agents providing performance that allows lubricating oil compositions to meet API CI-4 requirements for use in cooled EGR equipped engines, such compounds do introduce both phosphorus into the engine that can poison the catalysts used in engine aftertreatment devices, as described supra.
Therefore, it would be advantageous to identify low phophorus API-CI-4 heavy duty diesel lubricating oil compositions that can be used in engines provided with cooled EGR systems, which lubricating oil compositions are also suitable for use in engines provided with aftertreatment devices containing oxidation and/or reduction catalysts.